This invention relates to electrochromic structures through which energy, including light, can be transmitted under controlled conditions, and more particularly, to large-area electrochromic structures for controlled energy transmission.
A variety of electrochromic materials are available for controlling the through-passage of energy. Such devices operate "chromatically" by producing a change in coloration in accordance with an applied stimulus or excitation.
Thus, a thermochromic device produces a color change in response to temperature. The resultant coloration of, for example, a sheet of material, depends upon the temperature to which the sheet is subjected. There is, however, the objection that a user has only limited control over the coloration since it is temperature dependent.
Another chromogenic device is "photochromic" in that its coloration depends upon the intensity of light that falls upon the device. When glass or other translucent materials are doped with iron oxide, a light-induced reaction produces increased darkening with increased intensity of light. Here again, the degree of coloration is dependent upon external effects.
Still another chromogenic device depends upon the extent to which an electrical stimulus is applied, and is said to be "electrochromic". In such devices the nature and extent of coloration depends upon an electrical effect.
Unfortunately, it has been difficult to produce economical large-scale chromogenic devices with useful optical properties. Thus, small-area devices have been produced for lenses (photochromic) and mirrors (electrochromic). However, it has been difficult to produce panels with large surfaces of the kind needed for vision applications such as windows and skylights.
Accordingly, it is an object of the invention to facilitate the manufacture and use of chromogenic structures. A related object is to facilitate large-scale chromogenic applications in which appreciable surface areas are required.
Still another object of the invention is to adapt chromogenic devices to light transmissive structures such as skylights and other arrangements that are intended to provide large-area control over natural illumination.
A further object of the invention is to facilitate the manufacture and use of electrochromic structures, particularly for relatively large-scale applications. In addition, the entrapment of radiant energy can produce a greenhouse effect. Accordingly, it is another object of the invention to provide electrochromic devices with a capability for reducing the extent to which sunlight and other forms of radiation produce solar heat gain in buildings and other structures.
Another object of the invention is to adapt electrochromic devices for widespread architectural and automotive applications, as well as aeronautic structures such as cockpits and avionic viewing surfaces.
Studies have been undertaken which relate various parameters of materials, and device configurations (e.g., area), to switching speed. Such studies are reported by J.P. Randine in Proc. SPIE IS4, 539 (1989) and Messrs. K-C. Ho, D.E. Singleton and C.B. Greenberg in Proc. Electrochem Soc. 90-2, 349 (1990). It has been predicted that as device area increases, the resistance of its transparent conducting layer is the major factor limiting switching time and uniformity of coloration or bleaching.
Accordingly, it is another object of the invention to counteract the increase in switching time and nonuniformity of coloration and bleaching in electrochromic devices as their areas increase.
One approach has been to add an electrically conducting grid structure in accordance with U.S. Pat. No. 4,768,865 of Greenberg and Singleton issued Sep. 6, 1988. This approach degrades the transmission of light and may be unsuitable where the objective is to attain high percentage transmission. The adverse effect of a grid structure can be significant, especially if multiple grids, one on top of each transparent conduction layer, are used to obtain the desired switching speed. It also is difficult to form grinds on non-planar surfaces.
According, a further object of the invention is to increase switching speeds for electrochromic structures and eliminate or minimize the use of electrical grids.
Accordingly, yet another object of the invention is to achieve fast switching, while maintaining a high light transmission, and relatively low cost and ease of manufacture.